Method for purification of indole derivative trimer, electrode active substance comprising the purified trimer, method for manufacturing the electrode active substance, and electrochemical cell using the same

ABSTRACT

The present invention provides a method for easily removing metal impurities from an indole derivative trimer containing the metal impurities, which method involves mixing the indole derivative trimer containing the metal impurities with an imidazole compound in a solvent comprising water under heating, and filtering and separating the indole derivative trimer, from which the metal impurities have been removed, from the obtained mixture. Further, mixing a protonic acid allows the doping of the indole derivative trimer to be conducted simultaneously with the metal impurity removal, whereby the indole derivative trimer can be used as an electrode active substance of an electrochemical cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for purification in which metal impurities are removed from a trimer of an indole derivative (hereinafter abbreviated as “indole derivative trimer”) containing the metal impurities, the indole derivative trimer being used for an electrode active substance of an electrochemical cell such as a secondary battery, an electric double layer capacitor, a redox capacitor or a capacitor. The present invention also relates to a method for manufacturing an electrode active substance in which doping to an indole derivative trimer and removal of transition metal impurities contained therein are simultaneously conducted. Further, the present invention relates to an electrode active substance comprising a high-purity indole derivative trimer manufactured thereby, and particularly to an electrochemical cell using the electrode active substance.

2. Related Art

Indole compounds are known to be usable for an electrode material of electrochemical cells in which protons can act as a charge carrier, and particularly indole derivative trimers are known to be a useful electrode active substance. Japanese Patent Laid-Open No. 2005-187393 (hereinafter reffered to as Patent Document 1) discloses that, among indole derivatives, a trimer of indole carboxylic ester has a capability of becoming an electrode active substance for an electrochemical cell having a sufficient electromotive force and capacity and excellent cycle characteristics. As manufacturing methods of such a trimer of the indole derivative, an electrolytic polymerization method as disclosed in J. Chem. Soc., Faraday Trans., 93(1997), p. 3791 (hereinafter reffered to as Non-patent Document 1) and a chemical polymerization method as disclosed in Specification of WO2002/032903 (hereinafter reffered to as Patent Document 2) are known.

Non-patent Document 1 reports on a synthesis of a trimer by an electrolytic reaction of unsubstituted indole or 5-cyanoindole. As the indole derivative trimers obtained by the method according to this document, a high-purity product is obtained because no oxidizing agent is used. But, it is difficult to manufacture the product in a large amount by the method, and a problem of being hardly applied as an industrially manufacturing method still remains.

On the other hand, the method described in Patent Document 2 relates to a manufacturing method which comprises adding a solution comprising at least one oxidizing agent, at least one organic solvent and water into a solution containing an indole derivative, and making them to react.

The method described in Patent Document 2 is one which can manufacture target indole derivative trimer industrially in a large amount. However, since a metal compound such as ferric chloride or cupric chloride is used as an oxidizing agent, the obtained trimer inevitably has mingled metal and halogen compounds and the like derived from the oxidizing agent in the polymerization procedure.

In using thus obtained trimer as an electrode active substance, a dopant anion is doped to the trimer. The doping process to the trimer is commonly conducted at ordinary temperature in a solution comprising a dopant anion such as dilute sulfuric acid. Although contained impurities are reduced a little at this stage, they are hardly said to be fully removed. When an indole derivative trimer containing particularly metal impurities as impurities is applied to an electrochemical cell as an electrode active substance, it causes the increase in leak current due to the side reaction of the impurities. Therefore, contained impurities, particularly metal impurities, are desirably removed as much as possible.

Although a method in which the metal impurities are eluted by treatment with a strong acid such as concentrated sulfuric acid or concentrated hydrochloric acid is generally known as the removing method of metal impurities, the treatment with these strong acids on an indole derivative trimer unfavorably causes a decrease in the performance as an electrode active substance.

Japanese Patent Laid-Open No.2005-154225 (hereinafter reffered to as Patent Document 3) proposes the use of a ligand compound forming a metal complex in place of the metal impurity removal by an acid. For a carbonaceous material comprising a nano-scale carbon tube or a nano-scale carbon tube including a transition metal or its alloy in the space inside the tube and a transition metal impurity, which is a target to be treated in this method, there is a problem in that if the acid treatment is conducted, even the transition metal or its alloy included partially in the space inside the tube of the nano-scale carbon tube is eluted, and the nano-scale carbon tube including the transition metal or its alloy in the space inside the tube ends up being denatured; then, Patent Document 3 proposes a method which does not use an acid.

However, the target indole derivative trimer of the present invention is an organic compound having nitrogen atoms, and whether or not metal impurities can be removed by the method described in Patent Document 3 is not at all known. Moreover, if the doping treatment with an acid such as dilute sulfuric acid can be conducted simultaneously with the impurity removal as described above, it is industrially very advantageous.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for purification which can easily remove metal impurities from an indole derivative trimer containing the metal impurities. The present invention is also directed to provide a method for manufacturing an electrode active substance containing a high-purity indole derivative trimer in which method doping of an indole derivative trimer and removal of contained metal impurities can simultaneously be conducted.

Further, the present invention has an object to provide an electrochemical cell excellent in leak current characteristics in an electrochemical cell such as a secondary battery, an electric double layer capacitor, a redox capacitor or a capacitor by using the highly-purified indole derivative trimer in which the metal impurities have been sufficiently removed as an electrode active substance.

As a result of-extensive studies to solve the above problems, the present inventors have found that, by mixing an indole derivative trimer containing metal impurities with an imidazole compound to form metal complexes in a solvent comprising water under heating and filtering and separating a liquid component containing the metal complexes and an indole derivative trimer, from which the metal impurities have been removed, the metal impurities are simply and effectively removed from the indole derivative trimer. The inventions have further found that by using a protonic acid together, a simultaneous doping can be conducted, and achieved the present invention.

Specifically, the present invention relates to a method for purification of an indole derivative trimer represented by the following general formula (1):

wherein, R each independently represents a hydrogen atom, hydroxy group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, aryloxy group, arylthio group, alkyl group, aryl group or heterocyclic compound residue, by removing a metal impurity contained therein, the method comprises:

mixing the indole derivative trimer containing a metal impurity with an imidazole compound to form a complex with the metal impurity in a solvent comprising at least water under heating; and

separating the indole derivative trimer, from which the metal impurity has been removed, from the obtained mixture.

Further, the present invention relates to a method for manufacturing an electrode active substance of an electrochemical cell which comprises an electrolyte comprising a proton source and can operate such that a proton acts as a charge carrier in a redox reaction involved in charge/discharge, the method comprises:

mixing an indole derivative trimer represented by the general formula (1) containing a metal impurity with an aqueous solution of a protonic acid comprising an anion of the same chemical species as an electrolyte used in an electrochemical cell under heating;

sequentially or simultaneously mixing the mixture with an imidazole compound to form a metal complex with the metal impurity; and

separating the indole derivative trimer, from which the metal impurity has been removed and in which the protonic acid anion has been doped, from the obtained mixture.

The present invention further relates to an electrode active substance obtained by the above-mentioned manufacturing method and comprising an indole derivative trimer preferably having a purity of not less than 95%.

Additionally, the present invention relates to an electrochemical cell which comprises an electrolyte containing a proton source and can operate such that protons act as a charge carrier in a redox reaction involved in charge/discharge, and to an electrochemical cell which comprises at least the above-mentioned electrode active substance as an electrode active substance.

According to the present invention, it becomes possible to effectively and also simply removing metal impurities from an indole derivative trimer containing the metal impurities.

Further, use of an electrode active substance for which removal of metal impurities and doping can be simultaneously conducted by mixing a protonic acid, allows for providing an electrochemical cell excellent in leak current characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an electrochemical element of an embodiment of the present invention; and

FIG. 2 is a sectional view of an electrochemical cell of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An indole derivative trimer to which the present invention is applied is manufactured by a method in which a transition metal compound is used as an oxidizing agent as described in the above-mentioned Patent Document 2 (WO2002/032903), and is represented by the following general formula (1):

wherein, R each independently represents a hydrogen atom, hydroxyl group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, aryloxy group, arylthio group, alkyl group, aryl group or heterocyclic compound residue.

In the above general formula (1), an acyl group includes an acyl group having 1 to 20 carbon atoms such as a formyl group, acetyl group or propionyl group.

A carboxylate ester group is represented by COOR′; and a sulfonate ester group is represented by SO₃R′. Herein, R′ is a linear or branched alkyl group having 1 to 6 carbon atoms.

An alkyl component in an alkyl group, alkoxyl group and alkylthio group is a linear, branched or cyclic alkyl group having 1 to 20 carbon atoms in total, which may have as a substituent a group exemplified as the above-mentioned R, i.e., a hydroxy group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, arylthio group, aryl group or heterocyclic group.

An aryl component in an aryl group, aryloxy group and arylthio group is an aryl group having 6 to 20 carbon atoms such as a phenyl group or naphthyl group, which may have as a substituent a group exemplified as the above-mentioned R, i.e., a hydroxy group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, arylthio group, alkyl group, aryl group or heterocyclic group.

A heterocyclic group is a cyclic group having 2 to 20 carbon atoms and comprising a nitrogen atom, oxygen atom, sulfur atom, etc., as a heteroatom, and includes, for example, groups of furyl, thienyl, pyrrolyl, imidazolyl, triazolyl, oxazolyl, thiazolyl, pyridyl and indolyl.

Among them, an indole derivative trimer having at least one carboxylate ester group in each indole unit is preferable.

An indole derivative trimer represented by the general formula (1) is manufactured by a chemical polymerization method using an oxidizing agent described in Patent Document 2, etc. from an indole monomer represented by the following general formula (2):

wherein, R each independently denotes the same meaning as R in the general formula (1).

An oxidizing agent includes, for example, ferric chloride hexahydrate, ferric chloride anhydride, ferric nitrate nonahydrate, ferric nitrate, ferric sulfate n-hydrate, ferric ammonium sulfate 12-hydrate, ferric perchlorate n-hydrate, ferric tetrafluoroborate, cupric chloride, cupric sulfate, cupric tetrafluoroborate, nitrosonium tetrafluoroborate, ammonium persulfate, sodium persulfate, potassium persulfate, potassium periodate, hydrogen peroxide, ozone, potassium ferricyanide, tetraammonium cerium(IV) dihydrate, bromine and iodine. Preferable is an iron compound such as ferric chloride hexahydrate, ferric chloride anhydride, ferric nitrate nonahydrate, ferric nitrate, ferric sulfate n-hydrate, ferric ammonium sulfate 12-hydrate, ferric perchlorate n-hydrate or ferric tetrafluoroborate, and this oxidizing agent can be used singly or in a combination of two or more in optional proportions.

The indole derivative trimer manufactured in such a manner contains metal impurities of several weight percents in terms of metal. Then, the present invention has an object to remove such metal impurities. The present invention also provides a method for conducting a doping treatment necessary for an electrode active substance simultaneously with the removal of metal impurities. Hereinafter, the basic mechanism of the method of the present invention will be described.

1. By dispersing an indole derivative trimer in a solvent comprising water (hereinafter, referred to as water-based solvent) under heating, metal impurities contained in the indole derivative trimer are eluted in the water-based solvent. At this time, by adding a protonic acid in the water-based solvent, the elution of the metal impurities is promoted, and an optional protonic anion can be doped in the indole derivative trimer.

2. Then or simultaneously, by mixing an imidazole compound, the eluted metal impurities and the imidazole compound form metal complexes.

3. The indole derivative trimer and the metal complexes dissolved in the reaction solvent are each separated from the reaction solution.

In the above 1, by raising the temperature of a mixture of an indole derivative trimer and a water-based solvent, or by adding a water-based solvent after heating an indole derivative trimer, aggregated indole derivative trimer particles can be homogeneously dispersed. Further, beforehand addition of a protonic acid in the water-based solvent allows homogeneous doping. Moreover, improvement in permeability of the protenic acid into the indole derivative trimer particles promotes elution of the contained metal impurities; even when a dilute acid such as a dilute sulfuric acid is used, a sufficient effect of removing the metal impurities can be obtained. Then, expansion of the particles occur involved in the permeation of the protonic acid, which makes a state that impurities such as unreacted raw materials physically adsorbed on the particles are easily released, and can then remove the impurities more efficiently.

Then, as shown in the above 2, by adding an imidazole compound, nitrogen of the imidazole coordinates to the eluted metals to form complexes. By complexing in such a manner, re-adhesion of the metals to the indole derivative trimer particles can be prevented, and since the obtained complexes can be present in the dissolved state in a solvent to be used, as shown in the above 3, they are easily separated from the indole derivative trimer particles by a simple separation method such as filtration.

In the present invention, solvent comprising water is used as the solvent. Solvents usable together with water are ones which can hold a dispersion state of an indole derivative trimer and have a compatibility with water. For example, alcohols and ketones are usable. As the solvent, use of substantially only water is preferable.

The protonic acid includes an inorganic acid such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid or hexafluorosilicic acid, and an organic acid such as a saturated monocarboxylic acid, an aliphatic carboxylic acid, an oxycarboxylic acid, p-toluenesulfonic acid, a polyvinylsulfonic acid or lauric acid. The protonic acid is preferably used as an aqueous solution of a concentration of not more than 30 weight %. If a high-concentration protonic acid is used, the performance of the indole derivative trimer may possibly degrade due to its structural degradation. The heat treatment can be conducted using only a solvent comprising water without using a protonic acid.

For conducting the doping necessary for an electrode active substance simultaneously with the removal of metal impurities, a protonic acid comprising an anion of the same species as an electrolyte used in an electrochemical cell is used. The doping can separately be conducted; the doping method includes a method in which after the treatment of an indole derivative trimer is conducted in a water-based solvent with no protonic acid added, the doping is conducted by an established method, or a method in which after the treatment thereof in a water-based solvent comprising a protonic acid and the de-doping of the protonic acid by an alkali, etc. are conducted, re-doping of a protonic acid comprising a prescribed dopant anion is conducted. There may be a case where an indole derivative trimer can be used without the doping in advance.

The imidazole compound to form a metal complex includes 2,2-biimidazole, 2-(2-pyridyl)imidazole and 1H-imidazole. Among them, 1H-imidazole is preferable. These imidazole compounds are excellent in the solubility in water and the coordination property to metals, and a metal-complexed imidazole compound and an unreacted imidazole compound are both dissolved in a water-based solvent, allowing easily separating an indole derivative trimer therefrom by filtration.

Mixing of an indole derivative trimer, and a water-based solvent or a water-based solvent comprising a protonic acid is conducted under heating. The heating temperature is not especially limited as long as the temperature is in the range of not more than the decomposition temperature of the indole derivative trimer. In the case where after an indole derivative trimer, and a water-based solvent or a water-based solvent comprising a protonic acid are mixed, the mixture is heated, the heating is preferably conducted in the temperature range from not less than 80° C. to not more than the reflux temperature of the mixture, preferably at the reflux temperature of the solvent. In the case where after an indole derivative trimer is heated in advance, a water-based solvent or a water-based solvent comprising a protonic acid is added, after the indole derivative trimer is heated in the temperature range from 100 to 300° C., preferably 180 to 270° C., a heated water-based solvent or water-based solvent comprising a protonic acid, preferably a solvent in the boiling state, is added thereto. The heating-holding time is not especially limited, but the treatment time is preferably set to be shorter with the heating temperature nearer to the decomposition temperature of the indole derivative trimer, and is desirably suitably set according to the heating temperature. If the heating is conducted for a long time at near the decomposition temperature, the performance of the indole derivative trimer may possibly degrade due to the structural degradation.

The mixing amount of a water-based solvent or a water-based solvent comprising a protonic acid is not especially limited as long as the indole derivative trimer is sufficiently wet, but generally, the amount is used in 1 to 50 times by weight that of the indole derivative trimer, preferably 5 to 30 times by weight, more preferably 10 to 20 times by weight.

Then, the addition and mixing of the imidazole compound are conducted preferably under heating. The mixing temperature is not especially limited as long as it is in the temperature range of not more than the boiling point of the imidazole compound. In the case where after an indole derivative trimer is heated in advance, a water-based solvent or a water-based solvent comprising a protonic acid is added, the imidazole compound may be added together with the water-based solvent or the water-based solvent comprising a protonic acid.

The using amount of the imidazole compound is not especially limited as long as the amount enough to react with contained metal impurities is used. Addition of an excessive amount of the imidazole compound has no problem. The treatment time after addition of the imidazole compound, as long as the time is enough to progress the complexing reaction of the contained metal impurities with the imidazole compound, can be suitably set according to the heating temperature condition and the adding amount of the imidazole compound.

After the reaction mixture has been treated in such a manner, the indole derivative trimer is separated from the reaction mixture by filtration. In filtering and separating, for enhancing the separation effect of the indole derivative trimer from water-soluble metal complexes or the imidazole compound remaining in the indole derivative trimer, washing may be conducted using water or an aqueous solution of a protonic acid . In the case where the aqueous solution of a protonic acid is used as the washing solvent, it preferably comprises the same protonic acid as that used in mixing under heating.

In the present invention, since an organic solvent, an oxidizing agent component, an unreacted raw material used in the process for producing an indole derivative trimer, low molecular-weight components, by-products, etc. are also removed other than metal impurities contained in an indole derivative trimer, an indole derivative trimer of a high purity, particularly a purity of not less than 95%, can be obtained.

Next, a constitution and a fabrication method of an electrochemical cell will be described.

The electrochemical cell of the present invention is an electrochemical cell which is constituted using an electrode active substance comprising the high-purity indole derivative trimer obtained by the method of the present invention, and comprises an electrolyte comprising a proton source, and in which a proton acts as a charge carrier in the redox reaction involved in charge/discharge.

The electrode active substance used in such an electrochemical cell may contain known electrode active substances having a redox property in a solution comprising a proton source as a proton-conductive compound in addition to the high-purity indole derivative trimer obtained by the method of the present invention. The known electrode active substances are not limited.

The known electrode active substances include, for example, a π-conjugated polymer such as a polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene vinylene, polyperynaphthalene, polyfuran, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole or a derivative thereof, a polymer comprising a hydroxyl group (a quinone oxygen is converted to a hydroxyl group by conjugation) such as a polyanthraquinone or polybenzoquinone, and a conductive polymer obtained by copolymerizing two or more monomers. These polymers are subjected to the doping to form a redox pair, thus developing the conductivity. These compounds are selectively used as a positive electrode active material and a negative electrode active material by suitably adjusting the difference between redox potentials thereof.

Especially in the present invention, it is preferable that an indole-derivative trimer represented by the general formula (1) is used as the positive electrode active substance, and a polyphenylquinoxaline derivative represented by the following general formula (3) is used as the negative electrode active substance:

wherein, R″ each individually denotes the similar meaning as R in the formula (1).

A constitutional view of an electrochemical element of the present invention is shown in FIG. 1; and a constitutional view of an electrochemical cell is shown in FIG. 2.

The constitution is such that a positive electrode 2 and a negative electrode 3, respectively, are formed on a positive electrode current collector 1 and on a negative electrode current collector 4, and these are laminated through an insulation layer (separator) 5. A proton only is involved as the charge carrier. An aqueous solution or nonaqueous solution comprising a proton source is filled as the electolyte solution, and sealed by a gasket 6 to fabricate an electrochemical element (FIG. 1). Then, a terminal plate 7 is provided each on the positive electrode side and the negative electrode side of the electrochemical element to fabricate an electrochemical cell constituted of one electrochemical element.

Herein, for the positive electrode 2 and the negative electrode 3 each, an active substance and a conductive auxiliary agent (e.g., a fibrous carbon (trade name: VGCF, manufactured by Showa Denko K.K.) or particulate carbon (trade name: KETJENBLACK EC60OJD, manufactured by KETJEN BLACK INTERNATIONAL COMPANY) of 1 to 50 parts by weight to the active substance, preferably 10 to 30 parts by weight, are mixed. The mixture powder is press-molded at ordinary temperature to 400° C., preferably 100 to 300° C. Alternatively, a slurry in which the mixture is dispersed in an optional organic solvent or water is prepared, optionally mixed with a binder of 1 to 20 parts by weight to the active substance, preferably 5 to 10 parts by weight, and printed and dried on a conductive base material by the screen printing to fabricate the electrodes.

The binder is not especially limited, but is preferably a fluorine-based polymer such as a polyvinilidene fluoride (PVdF) or polytetrafluoroethylene (PTFE). Its molecular weight is usable with no especial limitation as long as it is in the range of dissolving in a solvent to be used.

As the electrolyte solution, an aqueous solution or a nonaqueous solution comprising a proton is used. For example, an acid is an organic acid or an inorganic acid, and specifically includes an inorganic acid such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid or hexafluorosilicic acid, and an organic acid such as a saturated monocarboxylic acid, an aliphatic carboxylic acid, oxycarboxylic acid, p-toluenesulfonic acid, a polyvinylsulfonic acid or lauric acid. The proton content is preferably 10⁻³ mol/l to 18 mol/l, more preferably 10⁻¹ mol/l to 7 mol/l.

The insulation layer (separator) 5 can be used with no especial limitation as long as it can electrically insulate between the positive electrode and the negative electrode of an electrochemical cell, and is a porous one which does not inhibit the migration of the electrolyte solution, particularly a proton. It includes, for example, a polyolefinic porous membrane and an ion exchange membrane. The thickness is not especially limited, but is commonly 10 to 200 μm, more preferably 10 to 80 μm.

The outer package shape of an electrochemical cell can be of a coin type, a laminate type, etc., and is not especially limited.

The electrochemical cell of the present invention is preferably one which can operate such that only a proton acts as the charge carrier in the redox reaction involved in charge/discharge, more specifically one which comprises an electrolyte comprising a proton source and can operate such that only adsorption/desorption of a proton of an electrode active substance is relevant to the electron transfer in the redox reaction involved in charge/discharge.

Hereinafter, the present invention will further specifically be described by way of examples, but the scope of the present invention is not limited to these examples only.

MANUFACTURE EXAMPLE 1 (Manufacture of an Indole Derivative Trimer)

In a three-neck flask of 200 μml, 10 μml of acetonitrile was charged; and 1.42 g of a methyl indole-6-carboxylate monomer was dissolved therein. On the other hand, for preparing an oxidizing agent solution, 16.2 g of ferric chloride anhydride and 5.4 g of water were added to and dissolved in 40 ml of acetonitrile, and agitated for 10 minutes. Then, the prepared oxidizing agent solution was added dropwise into the acetonitrile solution of the methyl indole-6-carboxylate monomer in 30 minutes, and thereafter agitated for 10 hours at 60° C. The reaction solution turned from pale yellow to green. The reaction was stopped at this point; the reaction solution was conducted to suction-filtration, washed with acetonitrile at ordinary temperature, then with methanol, and dried in the air at 120° C. for 5 hours to obtain 1.12 9 of trimethyl 6,11-dihydro-5H-diindolo[2,3-a:2′,3′-c]carbazole-3,8, 13-tricarboxylate (methyl indole-6-carboxylate trimer) as a green powder.

The purity of the obtained indole derivative trimer was calculated by HPLC analysis from its area count, and the metal impurity amount was calculated by IPC emission spectrometry. These results are shown in Table 1.

EXAMPLE 1

About 1 g of the trimer obtained in Manufacture Example 1 was transferred to a beaker of 200 ml; 15 g of a 5 wt. %sulfuric acid aqueous solution as the solvent was charged therein; the mixture solvent was agitated at ordinary temperature for 5 minutes; and the heater temperature was raised to 180° C. and held for 30 minutes. The mixture solvent was mixed with 8 g of 1H-imidazole, and further held at the same temperature for 10 minutes with agitating. Thereafter, the trimer was separated from the solvent by suction-filtration; and the filtered trimer was slightly washed with boiling water, and dried to obtain pale-green crystals. The filtrate was yellow.

EXAMPLE 2

Example 2 was performed as in Example 1, but using a sulfuric acid aqueous solution of 20 wt. %as the solvent.

EXAMPLE 3

Example 3 was performed as in Example 1, but using a sulfuric acid aqueous solution of 40 wt. %as the solvent.

EXAMPLE 4

Example 4 was performed as in Example 1, but using a hexafluorophosphoric acid aqueous solution of 20 wt. %as the solvent.

EXAMPLE 5

Example 5 was performed as in Example 1, but using an ion exchange water as the solvent.

EXAMPLE 6

About 1 g of the trimer obtained in Manufacture Example 1 was heated at 260° C.; a mixture solvent composed of 15 g of a 20 wt.% hexafluorophosphoric aqueous solution and 8 g of 1H-imidazole was boiled, mixed with the heated trimer, and agitated at a heater temperature of 180° C. for 5 minutes. Thereafter, the mixture was, as in Example 1, filtered, washed, and dried to obtain pale-blue crystals.

EXAMPLE 7

Example 7 was performed as in Example 1, but with the heater temperature set at 80° C.

COMPARATIVE EXAMPLE 1

About 1 g of the trimer manufactured in Manufacture Example 1 was transferred to a beaker of 200 ml; 15 g of a 20 wt. %sulfuric acid aqueous solution as the solvent was charged therein, agitated for 30 minutes at ordinary temperature, filtered and separated, and dried to obtain dark green crystals.

The purities of trimers and the amounts of metal impurities in the crystals obtained in the above Examples 1 to 7 and Comparative Example 1 were measured as described above. The dopant amounts were calculated by the ion chromatography analysis. The results are shown together in Table 1.

(Resistivity Measurement)

The crystals obtained in Examples 1, 2, 3, 5 and 7 and Comparative Example 1 were each press-molded into an electrode of 200 μm in thickness; and the electrode resistivities were measured using a four-terminal resistance measuring device. The results are shown together in Table 1.

(CV Test)

The CV tests of the crystals obtained in Examples 1, 2, 3, 5 and 7 and Comparative Example 1 were performed to measure the CV capacities.

Each test sample was manufactured by mixing the corresponding crystals and the fibrous carbon VGCF as the conductive auxiliary agent in a weight ratio of 7:3, adding dimethylformamide (DMF) to the mixture to make a past-like substance, coating on a 50mm×5mm carbon sheet, and drying the sheet at 120° C. for 1 hour.

The obtained each test sample was immersed in a 20 wt. %sulfuric acid aqueous solution, and subjected to a measurement of the cyclic voltammetry (CV) curve under conditions of a sweeping potential of 200 to 1,200 mV and a sweeping rate of 20 mV/s. An Ag/AgCI electrode was used as the reference electrode; and platinum was used as the counter electrode.

(Fabrication of an Electrochemical Cell, and Leak Current Characteristics Test)

As each positive electrode active substance, the corresponding crystals manufactured in Examples 1, 2, 3, 5 and 7 and Comparative Example 1 were used, and fibrous carbon VGCF and a polyvinylidene fluoride (average molecular weight of 1,100) were selected as a conductive auxiliary agent and a binder, respectively. These were adjusted so as to become 69:23:8 in weight ratio in the description order, and agitated and mixed by a blender. The mixture powder was charged in a mold of 10 mm square and press-molded at 200° C. for 1 minute to obtain each corresponding positive electrode.

For a negative electrode, a polyphenylquinoxaline of a proton-conductive polymer as the active substance was used, and the conductive carbon (trade name: KETJENBLACK EC600JD) were selected as the conductive auxiliary agent. These were adjusted so as to become 75:25 in weight ratio in the description order, and agitated and mixed by a blender. The mixture powder was charged in a mold of 10 mm in square and press-molded at 300° C. for 2 minutes to obtain the negative electrode.

As an electrolyte solution, a 20 wt. %sulfuric acid aqueous solution was used. A separator in which a porous nonwoven fabric of 50 μm in thickness was impregnated with the electrolyte solution was used.

The obtained positive electrode and negative electrode were laminated with the electrode surfaces facing each other through the separator, and armored with a gasket to fabricate an electrochemical element; and terminal plates were provided on both sides of the positive electrode and the negative electrode to fabricate an electrochemical cell.

With respect to the electrochemical cell thus obtained, the leak current of the cell was measured. The measurement was performed at 60° C. under a charging condition of CCCV: 1 mA-2.5 V, 24 h, and the current value at the charging-finish time was measured.

The results are shown together in Table 1. TABLE 1 Protonic acid Fe impurity Dopant CV Leak Purity concentration amount amount Resistivity capacity current (%) (wt %) (wt %) (wt %) (Ω · cm) (C/g) (μA) Manufacture 82.7 — 5.43 — — — — Example Example 1 98.2 5 0.07 1.26(SO₄ ²⁻) 1.2 × 10⁻³ 274.5 0.24 Example 2 95.9 20 0.13 6.84(SO₄ ²⁻) 5.5 × 10⁻² 251.6 0.57 Example 3 92.7 40 0.11 12.56(SO₄ ²⁻)  5.0 × 10⁻² 176.4 0.79 Example 4 97.0 20 0 3.71(PF₆ ⁻)  — — — Example 5 95.1 0 0.13 — 2.1 × 10⁻⁵ 245.4 0.57 Example 6 97.8 20 0.03 3.24(PF₆ ⁻)  — — — Example 7 91.7 5 0.68 1.89(SO₄ ²⁻) 1.1 × 10⁻³ 211.8 2.19 Comparative 90.6 20 2.46 5.43(SO₄ ²⁻) 7.2 × 10⁻² 206.7 11.2 Example 1

As is clear from Table 1, in comparison with Comparative Example 1, which was subjected only to the acid treatment at ordinary temperature (25° C.), in the Examples of the present invention, the purity of the trimer is improved, and the metal impurity is remarkably reduced.

Except Example 3, correlated with this analysis result, the increase in the CV capacity and the remarkable decrease in the leak current of the electrochemical cell are observed.

From the analysis result of each Example, the anion of the used protonic acid is observed to be doped; from Examples 1, 2, 3, 5 and 7, the doping amounts and the resistivities are observed to vary depending on the used protonic acid concentrations; and the optional protonic acid anion-doped high-purity indole derivative trimer is thus confirmed to be obtained.

From the results of Examples 1 to 3, that the concentration of a protonic acid is not more than 30 wt. %is found to provide a favorable result. In the case of Example 3 where a protonic acid of 40 wt. %is used, the decrease in the CV capacity is observed, resulting in implying a structural deterioration of the indole derivative trimer.

As shown in Example 5, the metal impurity amount is confirmed to be decreased without using a protonic acid.

From Examples 2 and 4, the high-purity indole derivative trimer is confirmed to be obtained not depending on the kind of a protonic acid.

From Examples 4 and 6, whether the trimer is heated after being mixed with the protonic acid or the protonic acid is added to the heated trimer, the metal impurity removal is confirmed to be similarly possible.

From Examples 1 to 7, the removal effect of the metal impurity is observed to have differences between the reaction temperatures, and a favorable removal effect is confirmed to be obtained in the case of not less than 100° C. 

1. A method for purification of an indole derivative trimer represented by the following general formula (1):

wherein, R each independently represents a hydrogen atom, hydroxy group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, aryloxy group, arylthio group, alkyl group, aryl group or heterocyclic compound residue, by removing a metal impurity contained therein, the method comprises: mixing the indole derivative trimer containing a metal impurity with an imidazole compound to form a complex with the metal impurity in a solvent comprising at least water under heating; and separating the indole derivative trimer, from which the metal impurity has been removed, from the obtained mixture.
 2. The method for purification according to claim 1, wherein the indole derivative trimer containing a metal impurity has been obtained by chemically polymerizing an indole derivative, represented by the following formula (2):

wherein, R each independently denotes the same meaning as R in the general formula (1), by using an oxidizing agent composed of a metal compound.
 3. The method for purification according to claim 1, wherein the indole derivative trimer containing a metal impurity is mixed with a protonic acid simultaneously with the imidazole compound or before the imidazole compound is mixed therewith.
 4. The method for purification according to claim 3, wherein the protonic acid is mixed as an aqueous solution of not more than 30 weight % in concentration.
 5. The method for purification according to claim 1, wherein the metal impurity contains iron as a metal component.
 6. The method for purification according to claim 1, wherein the imidazole compound is 1H-imidazole.
 7. The method for purification according to claim 1, wherein a mixture of the indole derivative trimer containing a metal impurity, the solvent comprising water and an imidazole compound and/or a protonic acid is subjected to a heat treatment in the temperature range from not less than 80° C. to not more than the reflux temperature of the mixture.
 8. The method for purification according to claim 1, wherein after the indole derivative trimer containing a metal impurity is heated in the temperature range from 100 to 300° C., the solvent comprising water and an imidazole compound and/or a protonic acid are mixed therewith.
 9. A method for manufacturing an electrode active substance of an electrochemical cell which comprises an electrolyte comprising a proton source and can operate such that a proton acts as a charge carrier in a redox reaction involved in charge/discharge, the method comprises: mixing an indole derivative trimer represented by the following general formula (1) containing a metal impurity with an aqueous solution of a protonic acid comprising an anion of the same chemical species as an electrolyte used in an electrochemical cell under heating, the formula (1) being:

wherein, R each independently represents a hydrogen atom, hydroxy group, carboxyl group, nitro group, vinyl group, halogen atom, acyl group, cyano group, amino group, trifluoromethyl group, sulfonyl group, sulfonic acid group, trifluoromethylthio group, carboxylate ester group, sulfonate ester group, alkoxyl group, alkylthio group, aryloxy group, arylthio group, alkyl group, aryl group or heterocyclic compound residue; sequentially or simultaneously mixing the mixture with an imidazole compound to form a metal complex with the metal impurity; and separating the indole derivative trimer, from which the metal impurity has been removed and in which the protonic acid anion has been doped, from the obtained mixture.
 10. The method for manufacturing an electrode active substance according to claim 9, wherein the indole derivative trimer containing a metal impurity has been obtained by chemically polymerizing an indole derivative, represented by the following formula (2):

wherein, R each independently denotes the same meaning as R in the general formula (1), by using an oxidizing agent composed of a metal compound.
 11. The method for manufacturing an electrode active substance according to claim 9, wherein the aqueous solution of the protonic acid has a concentration of not more than 30 weight %.
 12. The method for manufacturing an electrode active substance according to claim 9, wherein the metal impurity contains iron as a metal component.
 13. The method for manufacturing an electrode active substance according to claim 9, wherein the imidazole compound is 1H-imidazole.
 14. The method for manufacturing an electrode active substance according to claim 9, wherein the mixture of the indole derivative trimer containing a metal impurity and the aqueous solution of the protonic acid or/and the imidazole compound is subjected to a heat treatment in the temperature range from not less than 80° C. to not more than the reflux temperature of the mixture.
 15. The method for manufacturing an electrode active substance according to claim 9, wherein after the indole derivative trimer containing a metal impurity is heated in the temperature range from 100 to 300° C., an aqueous solution of the protonic acid or/and an imidazole compound are mixed therewith.
 16. An electrode active substance comprising an indole derivative trimer obtained by the method for manufacturing according to claim
 9. 17. The electrode active substance according to claim 16, wherein the indole derivative trimer has a purity of not less than 95%.
 18. An electrochemical cell comprising an electrolyte containing a proton source and capable of operating such that a proton acts as a charge carrier in a redox reaction involved in charge/discharge, wherein the electrochemical cell comprises at least the electrode active substance according to claim
 16. 19. An electrochemical cell comprising an electrolyte comprising a proton source and capable of operating such that a proton acts as a charge carrier in a redox reaction involved in charge/discharge, wherein the electrochemical cell comprises at least the electrode active substance according to claim
 17. 